1. Field of the Invention
This invention relates to toothed power transmission belts and, more particularly, to a toothed belt with a canvas cover thereon which accounts for excellent dimensional stability and resistance to abrasion in use.
2. Background Art
Toothed power transmission belts are used in many diverse environments. The toothed belt is particularly valuable in environments in which it is required to operate under heavy load, at high speeds and at high temperatures. Under such conditions, the teeth are prone to deterioration. The severe shearing stresses on the teeth often result in crack generation and severance of the teeth, which may render the entire belt unusable.
To alleviate the above problems, it is known to place a canvas cover over the belt teeth. It is known to form the canvas using relatively inextensible yarn for the warp, which yarns align widthwise of the belt. It is also known to use weft yarns having woolie finished contracted nylon fibers with the weft yarns aligning in the lengthwise direction of the belt.
During woolie processing, the denier of the yarns affects the contraction rate. This contraction is significant particularly with thin filaments. It is known in the art, to control the contraction rate, to use multi-filaments of about 40 denier which are defined by accumulated filaments, each of three to four denier.
While the advantages of improved resistance to tooth shearing and abrasion are enhanced up to a certain point by increasing the number and thickness of the twisted yarns, above a certain point there is a detrimental affect that results. Excessive yarn size and number may detrimentally alter the PLD value of the belt. The PLD is equal to the distance between the center of the load carrying cords of a belt and the bottom surface of the groove between adjacent, longitudinally spaced teeth. The PLD value is a critical dimension that must be controlled to insure a sufficient engagement between the toothed portions of the belts and the cooperating pulleys.
For the above reasons, it has been proposed in the art to use a canvas cover yarn in which a) the weft is formed by twisting yarn of not less than 210d with a twisting coefficient of not less than 1.5 and b) the warp is formed of a twisted yarn or mono-filament yarn, with the warp and weft woven with a cover factor of not greater than 10. Such an arrangement is disclosed in Japanese Patent Laid-Open No. 62-7414.
In conventional toothed belts, the pressure on the canvas cover, and consequently the critical PLD value, depends on a variety of factors, including: a) the pressure exerted by the tension members/load carrying cords on the canvas cover; b) the tension member diameter; c) the tension member winding pitch; and the like. For example, in an exemplary belt, the warp yarns in the adhesive rubber layer of the belt reside outside of the load carrying cords. During manufacture, the load carrying cords, when wrapped as around a forming drum, cause an exaggerated wavy pattern for the warp yarns. This results in an increase in the PLD value.
Due to the above variables, belts constructed according to prior art techniques have generally had inconsistent and oft times unpredictable PLD values.